1. Field of the Invention
The present invention relates to a resource control method in a mobile communication system using a resource reservation protocol (RSVP).
2. Description of the Related Art
With rapid growth of the mobile communication technique in recent years, the data transfer rate in a radio section has been greatly increased. Besides conventional service, such as speech or low rate packet communication, therefore, delivery of a large capacity of real time data, such as video data or music data is about to become possible. It is considered that a wide variety of services using large capacity communication techniques will be provided. For implementing large capacity service while ensuring its quality, it is demanded to ensure a requested quality (QoS) not only in radio sections but also in all nodes in a network.
The resource reservation protocol (RSVP) is one of protocols for ensuring QoS at the time of communication. IETF (Internet Engineering Task Force) is prescribed by RFC 2205. In the RSVP, a unit of communication in a certain end-to-end is defined as session and a data stream in each session is defined as flow. The RSVP is a protocol for previously ensuring network resources, such as abandwidth, in all nodes in a network through which a flow passes. A flow reception system exchanges RSVP messages with a transmission system. In this process, resources on all nodes (such as routers in an IP network) that exist between the reception system and the transmission system are reserved. Furthermore, in the RSVP, reservation states in each node are managed by using an “RSVP soft state” so as to be capable of flexibly coping with a dynamic network such as an IP network. The RSVP soft state is always updated to the latest state by a periodically issued refresh message. The RSVP is used mainly in IP networks. There are no examples of use of RSVP in mobile communication systems. Concrete implementation methods and problems are not yet made clear in many respects.
Supposing that the RSVP is applied to a current mobile communication system, a system example is schematically shown in FIG. 1. A mobile terminal 106 accesses a server 100 via a core network (CN) 101 and a radio access network (RAN) 102 formed of radio network nodes 103 to 105, and conducts data down loading. The RSVP is mounted on each of nodes in the core network 101. And the RSVP is mounted on each of the radio network nodes 103 to 105 as well. An operation conducted in such a system from the time when the mobile terminal 106 starts downloading of data from the server 100 until the downloading is finished will now be described briefly.
FIG. 2 is a diagram showing a processing sequence of the RSVP. FIG. 2 shows how a mobile terminal downloads data delivered from a server. Processing concerning the RSVP can be divided into the following three main classes:
(1) resource reservation processing on download path at the time of session start;
(2) refresh operation of soft state in downloading;
(3) resource deletion processing at the time of session end.
At the time of session start, the mobile terminal conducts processing for participating in a data delivery group by using a suitable protocol. Upon accepting this processing, the server transmits a RSVP message called “PATH” that stores route information (path reservation contents) concerning a flow to the mobile terminal. The PATH message is decoded on all RSVP nodes (nodes mounting the RSVP in the core network 101 and radio network nodes mounting the RSVP shown in FIG. 1) on the way to the mobile terminal to establish a PATH state (path reservation state), which is one of RSVP soft states, on the RSVP nodes. Upon receiving the PATH message, the mobile terminal transmits a RSVP message called “RESV” that stores information (resource reservation contents) concerning resource reservations to the server. In the same way as the PATH message, the RESV message is decoded on all RSVP nodes on the way to the server to establish a RESV state (resource reservation state), which is one of RSVP soft states, on the RSVP nodes. In this way, exchange of the PATH message and the RESV message is conducted between the mobile terminal and the server. As a result, resource reservations on all nodes that exist on the path between the mobile terminal and the server are completed.
Upon completion of resource reservations, the server starts data transmission to the mobile terminal. During that time, the RSVP resource reservation state (RSVP soft state) on each node is always updated (refreshed) to the latest information. On the basis of a timer (refresh timer) determined by the RSVP, therefore, the server periodically transmits the PATH message to the mobile terminal, and the mobile terminal periodically transmits the RESV message to the server. Even if the flow route is dynamically changed by a movement of the mobile terminal or a failure of a network node or a resource reservation request (resource reservation contents) is altered by the mobile terminal, therefore, the resource reservation state can always follow it.
In the case where the session is finished, the mobile terminal sends a RSVP message called “RESV TEAR” to the server and the server sends a RSVP message called “PATH TEAR” to the mobile terminal. Upon receiving the RESV TEAR message and the PATH TEAR message, each node on the session conducts deletion processing of the resource reservation state and the path state on the node.
Although not shown in FIG. 2, it is not always necessary to transmit the RESV TEAR message and the PATH TEAR message at the time of end of the session, because the reservation state of each node is deleted autonomously if a refresh message is not received within a predetermined time.
As described above, the resource reservation state in each node on the path (flow) is always updated to its latest state. This aims at always ensuring optimum QoS with respect to the data flow in a dynamically changing network environment. At fixed time intervals or whenever the reservation state changes, therefore, the server and the mobile terminal must transmit a PATH message or a RESV message (which may also be collectively referred to as “refresh message”). In the mobile communication system, the refresh operation especially poses a problem.
The property of the mobile communication system differs from that of the ordinary communication system in that the position of the terminal device (mobile terminal) is frequently changed. In other words, if the mobile terminal changes its position, a change of the path route or a change of the radio state occurs. Even after resources required in the network have been reserved between the mobile terminal and the server, therefore, the reservation state must be frequently refreshed according to a state change caused by a movement of the mobile terminal. In addition, as described above, the exchange of the refresh message must be conducted steadily even in the case where the reservation state does not change.
In the mobile communication system thus expected to frequently change in communication route and radio state, the transmission interval of the refresh messages must be extremely short for the resource reservation state to follow the changes in these states.
For these reasons, a great deal of refresh messages are exchanged between the mobile terminal and the server shown in FIG. 1. Thereupon, there is a possibility that the refresh operation will exert pressure upon resources of the core network and the mobile communication network.
Furthermore, data delivery nodes (data transmission/reception nodes) such as servers typically exist in the core network. In such a case, it may occur that the reservation state is actually altered in only a portion near a mobile terminal even if refresh messages are frequently exchanged between the server and the mobile terminal. In this case, the reservation state in each node in the core network is not altered at all, and only the same reservation state is simply updated, because the topology in the core network is considered not to change frequently as compared with the radio access network. Even if each node in the core network receives refresh messages, therefore, network resources are merely consumed wastefully.
In other words, if the refresh period in the resource reservation state in the mobile communication system shown in FIG. 1 is set to be short so as to conform to the radio access network, the reservation state can properly follow the state change of the radio access network. However, wasteful refresh messages flow into the core network, and waste network resources (see FIG. 3(A)). On the other hand, if the refresh period is set to be long in conformity with the core network, wasteful refresh messages reduce in the core network. However, it becomes impossible for the reservation state to follow the state change of the radio access network (see FIG. 3(B)).
As heretofore described, if it is attempted to mount the RSVP in the mobile communication system, it is necessary to satisfy two contradictory requests simultaneously. In other words, on a side located near the mobile terminal on the same path, it is necessary to frequently exchange refresh messages due to the mobility of the mobile terminal. On the other hand, on a side located near the server, it is necessary to, on the contrary, suppress refresh messages to minimum and save network resources.